Can you provide an example from the scriptures of Udi curing an incurable disease?

Yes, Chapter 34 of the Shri Sai Satcharitra provides a powerful example. It tells the story of a doctor from Malegaon whose young nephew suffered from a bone-cyst, also known as Hadyavarn. Despite the doctor's own expertise and the efforts of other intelligent surgeons and physicians who tried all native and foreign treatments, the ailment showed no improvement. The boy was in great distress. As a last resort, the family heard about Sai Maharaj of Shirdi and his Udi. They traveled to Shirdi, and Sai, the embodiment of compassion, gave them Udi, instructing them to smear it on the wound and to be worry-free. This story serves as a prime illustration of how Udi can cure even the most difficult maladies.